The present invention pertains to an apparatus for heating a rotary cylindrical roll and, more particularly, to an apparatus for heating a roll with steam and for handling the removal of condensate from a steam heated roll.
Rotary cylindrical drums and rolls are used in a wide variety of material treating applications. In one particularly common use, webs of material to be treated are wrapped around a heated rotary roll which transmits heat to the web traveling thereon. Steam is the most commonly used heating fluid and steam heated rolls are well known in the art. Steam is typically supplied and condensate water removed from the interior of the roll via axial bores in the roll shaft and utilizing rotary joints for the steam supply and siphon tubes for condensate withdrawal. The steam may be supplied to the entire open interior of the roll or may be directed to channels formed in the interior cylindrical wall of the roll. Condensate removal may be effected with a non-rotating siphon tube with an inlet positioned near the interior of the roll shell at the lowermost point of roll rotation, or with radially extending condensate removal tubes which extend from the roll shell to a common condensate outlet in the roll shaft.
At least two problems which are directly related to uneven roll heating and resultant thermal distortion have long plagued the industry. Condensate in the roll interior will accumulate by gravity flow in the lowermost region of the roll, except when the roll is turning at a high enough speed so that centrifugal force overcomes the force of gravity and the condensate is spread in a thin layer on the entire cylindrical interior of the drum, a condition sometimes referred to as "rimming". However, at low speed or when the roll is stopped, the condensate may pool to a depth sufficient to insulate the metal roll shell from direct contact by steam and, as a result, the lower portion of the roll shell will be much cooler than the remainder of the roll shell. The cooler portion of the roll shell will tend to bow inwardly with potentially serious consequences. For example, in a steam heated corrugating roll of the type used in the manufacture of single face corrugated paperboard, the bowing resulting from thermal distortion may cause the paper web to be cut by greatly varying nip forces when roll rotation is commenced.
In steam heated rolls, if the roll end walls (or end bells) are not maintained close to the operating temperature of the heated cylindrical drum wall, thermal distortion may result in roll shell separation from the end walls.
Another problem in steam heating systems for rolls is the inevitable presence of non-condensible gases in the steam system occurring, for example, from unavoidable air leakage and the like. If such non-condensible gases are not flushed from the system, they will diminish the rate of condensate formation and, therefore, the heat transfer capability of the steam. A flow-through system for supplying steam and utilizing steam pressure to return the condensate would allow the non-condensible gases to be flushed for removal outside the roll utilizing removal means, such as a flash tank, well known in the art.
In accordance with the present invention, a steam heated roll utilizes a flow-through steam supply and condensate return system which addresses and solves all of the problems attendant prior art rolls.